The present invention relates to a valve device of the plug valve type. This type of valve has a plug which closes the duct of a pipe when the valve is in the closed position and which has an opening for co-operating with said duct when the valve is in the open position.
Amongst conventional plug type valve devices, the use of a valve having a plug which is spherical, for example, is widespread, in particular because of the simplicity with which it can be made and because of its reliability. FIG. 11A is a section view showing a spherical or ball valve device in the closed position. Sealing between a plug 500 and a valve body (not shown) is provided by an annular gasket 501 which co-operates with both of those two elements. In this position, a fluid flow opening 502 in the plug lies substantially perpendicular to the fluid flow direction represented by arrow C, i.e. away from the passage defined by the gasket 501. FIG. 11B shows the same valve device in an open position. In this position, the opening 502 is in alignment with the fluid flow direction along the duct, thus allowing fluid to flow from upstream to downstream in the duct.
For a ball valve as described above, FIG. 12 gives the hydraulic characteristic, i.e. the head loss coefficient of the valve as a function of the angle to which its plug is opened. From this figure, it can be seen that the appearance of the head loss curve 90 is characterized by a sudden drop in head loss coefficient during the initial instants of opening. The head loss coefficient K is expressed by the following relationship:   K  =            ρ      ⁢              xe2x80x83            ⁢      Δ      ⁢              xe2x80x83            ⁢      P              Qm      2      
where r is the density of the fluid, DP the head loss, and Qm the mass flow rate.
Thus, with that type of configuration, it is difficult to control head loss accurately while opening the plug, and as a result fluid flow rate is characterized solely by two states with practically no transition between them: an OFF state (maximum head loss) in the closed position so long as the opening in the plug has not moved beyond the gasket, and an ON state (minimum head loss) where the head loss coefficient drops suddenly as soon as the opening in the plug begins to go beyond the gasket.
The present invention seeks to remedy the above-mentioned drawbacks and to provide a plug type valve device which enables its hydraulic characteristics to be controlled so as to obtain a plurality of flow stages.
These objects are achieved by a valve device comprising a valve body defining fluid flow duct, said duct having an upstream portion and a downstream portion between which there is disposed a plug having a main flow opening, the plug being movable between a first position in which the main flow opening co-operates with the flow duct, and a second position in which the plug closes said duct, the device being characterized in that it has a single annular gasket providing sealing between the duct and the outside wall of the plug, and in that said plug further includes at least one secondary flow means (also called secondary flow arrangement herein) offset relative to said main flow opening so that when said plug is in an intermediate position between the open and closed positions, fluid flows along the duct by penetrating directly through the main flow opening beside the upstream portion of the duct and leaving via said secondary flow means beside the downstream portion of the duct.
Thus, the present invention proposes a valve device which makes it simple and inexpensive to adapt the hydraulic characteristic of a plug type valve so as to obtain a plurality of flow stages.
The annular gasket may be placed either between the outside wall of the plug and the downstream portion of the duct, or between the outside wall of the plug and the upstream portion of the duct.
More specifically, the secondary flow means are of shape and size that are calibrated as a function of the desired level of head loss.
In this way, the level of head loss in each flow stage can be determined by the shape and size of the secondary flow means.
More particularly, the value of the spacing between said secondary flow means and said main flow opening in the travel plane of the plug is a function of the duration of the flow stage produced via the secondary flow means.
Consequently, during displacement of the plug between a closed position to an open position, or vice versa, the length of the flow stage produced by the secondary flow means can be controlled by the value of the spacing between said means and the main opening.
In a second embodiment of the invention, the secondary flow means comprise an opening placed between the outside surface of the plug and the cavity defined by the main flow opening.
In another embodiment of the invention, the secondary flow means comprises first and second openings disposed between the outside surface of the plug and the cavity defined by the main flow opening, the first and second openings being substantially in alignment on an axis defined by the diameter of the plug.
In yet another embodiment of the invention, the secondary flow means comprises at least one opening offset by a length parallel to the main flow opening of the plug.
Still in another embodiment of the invention, the secondary flow means comprises a plurality of openings disposed between the outside wall or surface of the plug and the cavity defined by the main flow opening.
In an aspect of the present invention, the secondary flow means is disposed in a plane that is inclined relative to the plane defined by the main flow opening.
In an aspect of the invention, the valve device has sealing means disposed between the outside surface of the plug and the duct.
In a particular aspect of the invention, the plug is spherical, cylindrical, or conical in shape.
According to a characteristic of the invention, the device includes drive means for turning the plug to move said plug between an open position and a closed position.
More particularly, the plug drive means turn said plug to rotate continuously in the same direction.
According to another characteristic of the invention, the plug moves in linear translation between an open position and a closed position.
The valve device as defined above may be applied to a liquid flow duct.